Channel waveguide-based evanescent-field optical sensors are developed to make a fully integrated chip biosensor. The optical system senses fluorescent analytes immobilized within a micrometric sized bioreactor well realized within an optical waveguide. The main novelty of this work is related to the fact that, within the bioreactor well, the excitation of the fluorescent signal is achieved by means of the evanescent field propagating through a silicon oxynitride waveguide. The immobilization of the emitting molecules is realized by functionalization of the waveguide surface by a wet chemical method. These photonic biosensors are successfully applied to detect low surface concentration (10−11 mol cm−2) of a green emitting organic dye. This approach could permit the selective detection of a wide range of chemical and biological species in complex matrices and can be exploited to set-up array-based screening devices. In this regard, the preferential excitation of the dye molecules in the close vicinity of the exposed waveguide core is also analysed.